1. Field of the Invention
This invention relates generally to implantable medical devices, and more particularly to methods, apparatus, and systems for monitoring power consumption and impedance characteristics relating to implantable medical devices.
2. Description of the Related Art
There have been many improvements over the last several decades in medical treatments for disorders of the nervous system, such as epilepsy and other motor disorders, and abnormal neural discharge disorders. One of the more recently available treatments involves the application of an electrical signal to reduce various symptoms or effects caused by such neural disorders. For example, electrical signals have been successfully applied at strategic locations in the human body to provide various benefits, including reducing occurrences of seizures and/or improving or ameliorating other conditions. A particular example of such a treatment regimen involves applying an electrical signal to the vagus nerve of the human body to reduce or eliminate epileptic seizures, as described in U.S. Pat. No. 4,702,254 to Dr. Jacob Zabara, which is hereby incorporated by reference in its entirety in this specification. Electrical stimulation of the vagus nerve may be provided by implanting an electrical device underneath the skin of a patient and performing a detection and electrical stimulation process. Alternatively, the system may operate without a detection system if the patient has been diagnosed with epilepsy, and may periodically apply a series of electrical pulses to the vagus (or other cranial) nerve intermittently throughout the day, or over another predetermined time interval.
Many types of implantable medical devices, such as pacemakers and drug infusion pumps, typically include custom integrated circuits that are complex, expensive, and specific to the intended use. These systems also typically employ proprietary communications techniques to transfer information between the implant and an external programmer. The custom circuitry is developed because of the need to keep power consumption at a minimum, to conform to the allowable size for implantable devices, and to support the complexity of the detection and communication techniques, while still supplying the particular intended therapy.
Typically, implantable medical devices (IMDs) involving the delivery of electrical pulses to body tissues, such as pacemakers (heart tissue) and vagus nerve stimulators (nerve tissue), comprise a pulse generator for generating the electrical pulses and a lead assembly coupled at its proximal end to the pulse generator terminals and at its distal end to one or more electrodes in contact with the body tissue to be stimulated. One of the key components of such IMDs is the power supply, ordinarily a battery, which may or may not be rechargeable. In many cases surgery is required to replace an exhausted battery. To provide adequate warning of impending depletion of the battery and subsequent degradation of the operation of the IMD, various signals may be established and monitored. One such signal is an elective replacement indicator (ERI) that may indicate that an electrical device component, such as a battery, has reached a point where replacement or recharging is recommended. Another indicator may be an end of service (EOS) signal, which may provide an indication that the operation of the implanted device is at, or near, termination and delivery of the intended therapy can no longer be guaranteed. ERI and EOS are commonly used indicators of the present status of an IMD battery. ERI is intended to be a warning signal of an impending EOS indication, providing sufficient time (e.g., several weeks or months) in typical applications to schedule and perform the replacement or recharging.
Generally, battery-powered IMDs base the EOS and the ERI signals on battery voltage and/or battery impedance measurements. One problem associated with these methodologies is that, for many battery chemistries, these measured battery characteristics do not have monotonically-changing values with respect to remaining charge. For example, lithium/carbon monofluoride (Li/CFx) cells commonly used in neurostimulators and other IMDs have a relatively flat voltage discharge curve for the majority of their charge life, and present status of the battery cannot be accurately and unambiguously determined from a measured battery characteristic.
Another problem associated with this methodology is the variability of current consumption for a specific device's programmed therapy or circuitry. This variability, combined with the uncertainty of the battery's present status prior to ERI or EOS, hinders reliable estimation of the anticipated time until reaching ERI or EOS. For scheduling purposes, it is desirable to have a constantly available and reliable estimate over all therapeutic parameter ranges and operation settings of the time until the device will reach EOS, and provide an indication, similar in purpose to ERI, when that time reaches a specific value or range.
Impedance measurements are used to assess the integrity of the electrical leads that deliver the stimulation provided by a pulse generator. A change in the impedance across the leads that deliver the electrical pulses may be indicative either of changes in a patient's body or in the electrical leads themselves. For example, damage in the lead, which may be induced by a break in one or more filaments in a multifilament lead wire, or changes in the body tissue where stimulation is delivered, may affect the efficacy of the stimulation therapy. Therefore, it is desirable for changes in the lead impedance, which may be indicative of various changes or malfunctions, to be accurately detected.
For instance, the integrity of the leads that deliver stimulation is of interest to insure that the proper therapy dosage is delivered to the patient. Some IMDs, most notably pacemakers, provide a voltage-controlled output that is delivered to one or more body locations (such as the heart). Other IMDs, such as a vagus nerve stimulator device developed by Cyberonics, Inc., provide a current-controlled output. Generally, however, state-of-the-art measurements of lead impedance involve an analysis of the delivery of a voltage signal from a capacitive (C) energy storage component through the resistive (R) lead impedance and an examination of the decay of that signal based upon a time-constant proportional to the product of the resistance and capacitance (RC). The total equivalent impedance present at the leads and the known energy source total equivalent capacitance cause a time-constant discharge curve. As the voltage on the capacitance is discharged through the resistance, the exponential decay of this voltage may be monitored to determine the decay time constant RC. From that time constant and an estimate of the known equivalent capacitance C, the equivalent resistance R presented by the leads may be mathematically estimated. However, this type of measurement may lead to inaccuracies for a number of reasons, including the fact that the discharging of the voltage signal may be affected by other resistances and capacitances in the system, the accuracy of the capacitor, the time, voltage, and algorithmic accuracies of the measurement system, and the like. It would be desirable to have a more efficient and accurate method, apparatus, and/or system to measure or assess the impedance present at the leads that deliver an electrical stimulation or therapy.
The present invention is directed to overcoming, or at least reducing, the effects of, one or more of the problems set forth above.